1. Field of the Invention
The present invention relates to an Ni based alloy having excellent strength, hardness, and toughness, a method for producing the Ni based alloy, and a forging die of the Ni based alloy.
2. Description of the Related Art
FIG. 5 shows a gear 1 to be used, for example, for an automobile transmission. The gear 1 has a large diameter section 2 and a small diameter section 3 which has a diameter smaller than that of the large diameter section 2. Outer teeth 4 are provided on a side circumferential wall of the small diameter section 3.
The gear 1 is produced, for example, by hot forging. At first, an unillustrated ring-shaped workpiece made of SCR420H, SCM420H, HNCM (according to JIS (Japanese Industrial Standard)) or the like, is heated to about 1100 to 1200° C. After that, the ring-shaped workpiece is arranged in a die. Subsequently, the workpiece is pressed by a punch or the like, and the workpiece is plastically deformed to have a shape corresponding to the gear 1. During this process, the outer teeth 4 are formed on the side circumferential wall of the ring-shaped workpiece by a teeth-forming section provided on the die. In the hot forging, the workpiece is softened by recrystallization. Therefore, no work hardening is caused. Accordingly, the ductility of the workpiece is increased, and hence the workpiece can be machined with ease.
Die steel for hot working including high speed tool steel and maraging stainless steel is widely used as a raw material of the die for hot forging, because the die steel for hot working is inexpensive and can be easily formed to various shapes.
When the gear 1 is produced by hot forging as described above, the temperature of the die is increased, because the heat is transmitted from the ring-shaped workpiece to the die. The temperature of the die is about 725° C., and instantaneously about 1100° C.
For this reason, when the hot forging is repeated about 3000 times, the die is abraded and chipped. If such a die is used, defective gears each having a size deviating from a predetermined standard are formed. Therefore, the forging machine is stopped, and then the die is replaced with a new die.
In this procedure, since the forging operation is interrupted, the production efficiency of the gear 1 is lowered. Further, the equipment cost for performing the hot forging is expensive, because the die is frequently replaced.
It is difficult to improve the production efficiency of forged products, because an ordinary hot forging die has a short service life. Therefore, the machining cost is expensive.